The Truth About Antenna Tuners, SWR, and Non-Resonant Antennas

Article at hamuniverse discussing the theory.

More, including lab-bench data backing up the previous link.  Data!  Real Data!  Can't argue with it.  Great graphic at the top of the article too.

And to wrap it all up, a how-to-use guide at the ARRL's site, so you know that's definitive.

TLDR version: Antenna tuners work well and are reasonably efficient.  Reflected power is not wasted, but turned right back around to go out through the antenna, no matter how non-resonant it may be.  The only significant losses are in the goings back-and-forth along the feed coax.  The rest of the RF power is radiated.  Contrary to somewhat popular belief, the returned power is not shunted to ground by the tuner.  Data to back this up is presented  in the second paper.

So go enjoy that random wire, that Zepp, that G5RV, and above all, enjoy that W3EDP, secure in the knowledge that you're not warming the worms in the ground, but getting useful signal out of an antenna that didn't cost you a mortgage payment.